Method and apparatus for measuring



Aug. 22, 1944. D. G. c. HARE METHOD AND APPARATUS FOR MEASURINGTHICKNESS Original Filed June 26. 1940 4 Sheets-Sheet l FIG. 2

DONALD G.C.HARE

XNVNTOR BY A HIS ATTGRNEIYS Aug. 22, 1944. D. G. c. HARE METHOD ANDAPPARATUS FOR MEASURING THICKNESS Original Filed June 26. 1940 4Sheets-Sheet 2 FIG. 4

FIG.9

FIG. 8

BY H l5 DONALD G.C.HARE

INVENTOR ATTORNEYS Aug. 22, 1944. D. G. c. HARE Re. 22,531

METHOD AND APPARATUS FOR MEASURING THICKNESS Original Filed June 26.1940 4 Sheets-Sheet 5 (n SCATTERED g INTENSITY 600- z (ARBUNITS) 6 it IN I q I- o g 500- j x N g 2 o 400 I I I I l I I O 5 l I 3O DISTANCE lNCM.

FIG. 7 CALIBRATION CURVE WALL THICKNESS VSI R.

AV. INTENSITY AT THICKNESS t. 0.6-1 WHERE R AV. INTENSITY FOR tI= 0.574

wAI I THICKNESS 0.3- I.

t (INCHES) m I 0.0 I I I I DONALD G.C. HARE INVENTOR ATTORN EY STAN DARDAug. 22, 1944. D. G. c. HARE 22531 METHOD AND APPARATUS FOR MEASURINGTHICKNESS Original Filed June 26. 1940 4 Sheets-Sheet 4 FIG. 11

"" IIII DQNALD G.C. HARE II INVENTOR z/kw MU" HIS ATTOR EYS Reiaued Aug.22, 1944 UNITED STATES PATENT OFFICE METHOD AND APPARATUS FOR MEASURINGTHICKNESS Original No. 2,277,756, dated March 31, 1942, Se-

rial No. 342,422, June 26, 1940.

Application for reissue April 22, 1944, Serial No. 532,318

Claims.

This invention relates to the measurement of thickness and particularlyto a method and an apparatus for measuring the thickness of the walls ofreceptacles or pipes adapted to contain or conduct liquids, such, forinstance, as the shells of oil stills or the walls of tubes adapted tocarry hydrocarbon oil through a heater.

The primary object of the invention is to provide a device which can beused for accurately determining the thickness of a wall from one sideonly without any necessity for obtaining access to the other side of thewall, and with which measurements can be made at a greater speed thanhas formerly been possible.

The methods of measuring the thickness of such materials as boiler ortubing walls may be arbitrarily separated into two groups; those whichrequire access to both sides of the wall to be measured, and thoserequiring access to only one side. Into the former group fall suchmethods as one type of calipering, examination by means of X-rays orgamma rays transmitted by the material, and certain types of magneticand electrical methods.

The second group includes magnetic and electrical methods and thosemethods based upon the assumption that the condition of the surface ofthe wall not accessible, is known. In this latter sub-group are includedthe calipering of the inside or outside of pipes or tubing, and thevisual examination of the inside of tubing by means of special opticalinstruments. Also to be included is the aural method, whereby thethickness is determined by the characteristic sound or tone created by atapping on the material with a suitable hammer. Since in most cases ofprimary interest it is not economically feasible to have access to bothsides of the material to be measured, the first group of methods willnot be discussed.

Certain inherent weaknesses in the methods of the prior art may bepointed out. The electrical methods are primarily those which measurethe resistance oi a portion of the wall under test. Since most materialsto be measured are metallic conductors-they possess a relatively highconductivity or low resistance. Thus, if a precise measurement isdesired, it is necessary to measure a small difference in a very lowresistance, a procedure difiicult to do even in a laboratory. In themagnetic method, use is made of either the permeability of the specimen,or of eddy currents generated in the specimen. These methods yieldprecise results only for very thin specimens, because of the very greateffect or the "surface layers of the material. However, the most seriousdifliculty with both electric and magnetic methods is that both dependto a large extent on the condition of strain and temperature of thematerial, and, particularly for the magnetic case, upon the physicalhistory of the specimen. The eflects due to these factors are not, as arule, regular, and in fact may be abrupt and very large.

If the interior of the tubing is accessible, one may determine theaverage wall thickness, as well as the presence of pitting, by suitableinside calipers, on the assumption that the condition of theinaccessible wall is known. However, no inside caliper measurement candetect a nonconcentric bore, 1. e., one in which the inner and outerwall surfaces, though circular, are not concentric, thus making one partof the wall thin compared to the average wall thickness. This averagethickness will be the thickness determined by caliper measurements, andmay be such that the wall thickness is apparently well within the safetylimits, when in fact one portion of the wall may be dangerously thin.Such cases are not rare, and in more than one instance the result hasbeen that tubing which when calipered appeared safe and was noted assuch, has later ruptured, with resulting disastrous fires.

The optical examination of tubing interiors has considerable value indetecting severe pitting due to corrosion or abrasion. The apparatus is.however, not convenient to use in any but the most ideally disposedtubing, and will yield little or no information regarding the uniformthinning of the walls.

The aural method, when used by a well trained expert, seems to becapable of a good degree of accuracy, particularly for such materials asboiler or tank shells. However, the relative number of cases to whichthis method may be applied is not large, and there is a most naturalindisposition to the trusting of the welfare of workers as well as ofthe investment in a method so patently dependent upon a highlyconditioned human reaction.

This invention comprises a new method and an apparatus capable ofmeasuring to a very high degree of precision the thickness of tubing orboiler walls, or other similar shells. The measurement requires accessto only one side of the wall, and yields information regarding thecondition of both sides. It may be used either inside or outside thetubing or other equipment or fixture,

and will work on non-metals as well as on metals. Its operation can bemade reasonably rapidcertainly as fast as the present caliperingmethods--and is quite independent of the physical history of thematerial, as well as of its present state of stress and strain. It canalso be adapted for use on elbows and bends of tubing.

In accordance with the invention, a device is provided having a casingwhich is adapted to be placed in contact with the surface of the plateor tube wall to be measured. A source or sources of penetratingradiation is housed within the casing in such a manner that theradiation will be preferably confined so as to be directed angularlytoward the surface of the wall. A device adapted to detect radiationwhich has been scattered and diffusely reflected within and by thematerial of the wall is associated with the casing and so positionedthat it will intercept some of the radiation so scattered and returnedoutwardly of the wall. The detecting deviceis preferably connected to asuitable instrument which can if desired be calibrated to read directlythe thickness of the wall being measured.

For a better understanding of the invention, reference may be had to thaccompanying drawings in which Figure 1 is a diagrammatic illustrationof the principles embodied in the invention;

Figure 2 is a perspective view of the device as positioned in contactwith the outside of a tube wall for measuring the thickness thereof;

Figure 3 is a bottomperspective view of the device;

Figure 4 is a sectional elevation through the device;

Figure 5 is a diagrammatic illustration of the device as used with astandard for calibration purposes;

Figure 6 is a curve developed for calibrating the device with a standardpipe;

Figure '7 is a curve obtained by comparing intensities due to variousthicknesses with the intensity from some arbitrary thickness chosen as astandard;

Figure 8 is a sectional elevation through a tube, the thickness of whichis to be measured and showing a modified form of the invention;

Figure 9 is a side sectional elevation taken on the line 9-9 of Figure8.

Figure 10 is an elevation through a section of pipe showing anothermodification of the device;

Figure 11 is a side sectional elevation taken on the line |l-H of Figure10;

Figure 12 is an elevation through a section of pipe showing stillanother modification of the invention;

Figure 13 is a side sectional elevation taken on the line l3-I3 ofFigure 12, and

Figure 14 is a sectional plan view taken on the broken linen-l4 ofFigure 12.

Briefly, this invention is based upon the well known physical principlethat any radiation having the properties of an electromagnetic wave suchas visible light, X-rays, gamma rays, and the like passing throughmatter will be scattered (a process similar to diffuse reflection, suchas the difiusion of light in a fog), and the amount of radiationscattered will increase with the amount of matter traversed. Thus, forexample, if one directs a beam of penetrating radiation such as gammarays upon a sheet of metal, a certain intensity will be scattered .inall directions, and one may even detect intensity scattered back towardthe source of the incident beam. Further, it will be shownthat thisscattered intensity will increase with the amount of material traversedby the incid nt radiation;

in this case, with the thickness of the metal sheet. The followingdiscussion, based upon elementary classical theory, will demonstratethis principle.

Referring to Figure 1, l0 represents a source of penetrating rays, suchas those gamma rays which are emitted by the elements of the radium,actinium or thorium series. This source is placed in a block I: ofmaterial such as lead, which will strongly absorb the emitted rays, sothat practically the only radiation from l0 which appears outside theblock is the narrow pencil of parallel rays which pass through the holell shown in block I I. This collimated beam impinges on and penetrates ablock ll, which may be of any material. It is well known to those versedin the art that when any electromagnetic radiation traverses matter, itwill, on the classical theory, set into forced vibration the electronsof the matter traversed, and that these electrons, being subject toperiodic accelerations, will themselves radiate energy. A good treatmentof this subject based upon classical consideration, is given by J. J.Thompson, who shows that if the intensity of the incident beam is Io,the intensity, Ie, scattered by a. single electron is given by where Ifwe have a small volume of scatterer containing a number of electrons dn,then, assuming that the electrons scatter independently, the

scattered intensity dis is e 111.: 0 cos 0)dn In any uniform materialthe electron density, 1. e., the number of electrons per unit volume isa constant hence, from (2) we see that the scattered intensity isproportional to the amount of scatterer irradiated by the primary orincident beam. This assumes that-neither the incident nor the scatteredbeam is absorbed in the scattering material-which is, of course, nottrue. However, we can, for the purpose of exposition of the method,neglect this factor, as it can be shown, by a treatment beyond the scopeof this disclosure, that, for reasonable thickness of seatterers, theeffect of absorption may be made of minor importance by suitablegeometrical consideratlon.

Referring again to Figure 1, we have here shown a primary ray or quantuml6, incident on a volume of scatterer whose cross section is that of thecollimated beam and whose length is dw. A scattered quantum I8 is shownincident on the detector 20-, which is some device su'ch as a.Geiger-Muller tube, ionization chamber, or photosensitive plate, whichwill detect the presence I of radiation of the nature of that emitted bysource In and scattered in block l4. Such devices are, when coupled withthe proper associated apparatus, commonly capable of determining thenumber of quanta incident per unit time, i. e., the intensity of theradiation incident upon them.

Ii we assume that the element of volume whose length is da: contains anumber of electrons dn,

the total scattered from this volume will be given by (2). Further. if:I: is reasonably small compared to the received by I. will be, to avery good approximation. independent of the a position of dz. Now thevolume of the scattering element of volume is late, where k is thecross-section of the incident beam. Then we may write irom (2) azpf tu-me and: (a

where In=intensity incident on detector II k'=a constant dependent onthe electron density oi block it and on the geometric relation of theblock and the detector ll Integrating (3) over a: from :o= to z=n Wethus see that, with certain elementary assumptions, the intensity ofscattered radiation \as detected by a detector in is proportional to thethickness of the scattering material. It is obvious that the detectorneed not be at 90 to the direction of the incident radiation. It is alsoobvious that the device need not be arranged so as to give a linearincrease in scattered intensity with thickness of scatterer, as long asthe actual relationship is known.

It should be emphasized that the elementary classical Equation 1 forscattering does not at all accurately describe the scattering of hardradiation such as gamma rays in so far as intensity and angulardistribution is concerned. However. even on the quantum-mechanicalbasis, the total scattered intensity increases with the amount ofscattering material traversed, and the exposition above set forth isqualitatively valid under quantum-mechanical consideration.

The deviceof Figures 2, 3 and 4 is one oi many possible arrangementswith which to utilize the above principle for making measurements oftubing wall thickness when one has access only to the exterior of thetubing. The source 22 may be, for this arrangement, any suitableradioactive material, such as the elements of the radium, actinium orthorium series, which may emit penetrating gamma rays. Use can also bemade of any of the substances normally nonradioactive but which becomemore or less temporarily radioactive after suitable treatment, such assodium which has been bombarded by neutrons oi suitable energy. Theprimary or incident radiation is collimated by the slot 24 in the leadblock 2', which confines the beam to desired limits. This lead shieldingalso protects the operator from the harmful eflects of radiation of thesource. The detector II of the scattered radiation may be aGeiger-Muller tube, ionization chamber, or other device suitable fordetecting the type of radiation utilized.

The bearings 30 shown are steel balls set in strips of brass oraluminum, which is fastened to the lead block. By using four properlydisposed balls the block may be made accurately selfaligning on a pipe,and yet oifer small resistance to translatory motion.

Figure 2 shows the block in position on a portion of tubing underexamination. In the cutaway section of the wall is depicted an incidentquantum 32 and a scattered quantum 34. This figure will make clear thatnearly any desired geometrical arrangement can be easily obtained (1 cos0):

length or detector 20, the intensity by proper choice of slot andposition of detector. The detector 28 is connected electrically by acable so of any convenient length to a direct current amplifier II. Thepower for this amplifier as well as the voltage for, the ionizationchamber or detector 28 is obtained from a suitable battery lil which maybe housed within the casing containing the amplifier ll. The currentoutput of the detector which, as has been described, is .a function ofthe thickness of the wall under examination is amplified and the outputof the amplifier 38 is indicated by the reading of the voltmeter I!shown as connected to the amplifier. Since the indication of thisvoltmeter then varies as the thickness of the wall being measured, asystem is provided which directly indicates the thickness of thespecimen under examination.

In Figures 8 through 13 are shown three forms oi the device arranged tomake measurements of tube wall thickness when access can be had only tothe interior of the tube.

In Figures 8 and 9, a. tube 50 is shown, the wall thickness of which itis desired to measure. A lead block or shield member 52 is provided witha slot 54 corresponding to the slot 24 of Figure 2 and at one end ofthis slot is disposed a source of radiation 56 corresponding to thesource 22 of Figure 2. Mounted in the lower portion of the block 52 is adetector 58 of scattered radiation, The device may be placed within andvmoved through the tube Bil by any suitable means. As shown, the block52 is attached to the end of a rod or pipe Bil long enough so that theblock and its associated elements can be manipulated within the tube.The electrical connections, not shown, from the detector 58 may passoutwardly of the tube through the pipe Gil.

The operation of this form of the device is substantially the same asthat described with respect to the form shown in Figures 2 through 4.The rays from the source lit are collimated by means of the slot 54 andenter the wall of the tube 50. Some of the rays scattered in the tubewall then pass to the detector 58 and the response of this detector maybe indicated by means of a suitable instrument such as is shown at It inFigure 2.

In Figures 10 and 11 is shown another form of the device for use withina tube 50a. This device is similar in general to that shown in Figures 8and 9 and comprises a lead block or shield member 52a provided with aslot 54a. A source of radiation 56a. is mounted within the block at oneend of the slot. A pair'of detectors "a are mounted at opposite sides ofthe open end of the slot 54a and the device is provided with a rod orpipe Gila by means of which it may be moved within a tube the walls ofwhich are to be measured. The operation is substantially the same asthat described with respect to Figures 8 and 9. the radiation from thesource 56a entering and being scattered within the wall of the tube 50aand some of the scattered radiation being picked up by the detectors5811 which are preferably connected electrically with an instrument suchas that disclosed at 38 in Figure 2.

Still another form of the device for use within a tube or pipe 50b isshown in Figures 12, 13 and 14. A lead block or shield member 52b isattached at one end of a suitable rod or pipe "b so that it can be movedwithin the tube 50b in contact with the inner surface of the wallthereof. The block 52b is provided with a slot 54b and at the inner endof the slot is mounted a source of radiation 56b similar to the source22 end of the slot lilb and receives radiation from the source ltb whichradiation has been scattered within the wall of the tube 50b. As is thecase with the forms shown in Figures 8 through 11, the detector 66b ispreferably connected elec-- trically by wires, not shown, with anindicating or recording instrument such as is shown at II in Figure 2.

While it is possible to calculate the amount of scattering which wouldbe detected from a given wall thickness, this is far from practical inmost cases. A more economical procedure is to calibrate the instrumentin terms of known tubing thicknesses. This may be done as shown inFigure by placing the device on different tubing thicknesses andplotting the obtained readings as a function of wall thickness. We maythus obtain a curve similar to Figure 6 showing the wall thickness atdifferent distances from the end of the pipe or tube. However, sucha'graph is a function of both the intensity of the source and thesensitivity of the recording system, and a better calibration curve isone of the type of Figure 7, which is a curve obtained by comparing theintensities due to various thicknesses to the intensity from somearbitrarythickness chosen as a standard. Such a curve is obviously. fora given instrument, independent of the source intensity and recordersensitivityat least as long as these factors do not vary durin a seriesof measurements. Having obtained such a calibration curve over thedesired range of thicknesses, the intensities recorded on measuring anypipe or tubing will immediately yield the thickness of the wall interms' of the standard thickness. It is in fact easily feasible tocalibrate the recorder to give readings directly in terms ofthicknesses. One may, of course, use a record ing meter which will makea permanent record on, say, a paper strip and this strip may bemechanically coupled to the measuring device so that the motion of thepaper corresponds to the motion of the device on the pipe beingmeasured; and the recorded meter deflection on the paper will form apermanent record of the wall thickness at the time of measurement. If itis desired to determine whether the tubing wall may be pitted orotherwise locally thinned, it may be necessary to make measurements atvarious positions on the circumference, or the device may be madesemi-circular or even circular, so as to examine a larger portion of thecircumference at one time. It must be pointed out, however, that if thedevice radiates the entire or major part of the circumference, thepossibility of detecting non-concentric bores is reduced.

It is obvious that the method can be made to work'equally well insidethe tubing, as well as on flat plates or boiler shells. In the case ofvery small tubes close together, or other cases where the space oneither side of the wall is very limited, the source and detector may beseparated and used in adjacent tubes, thus determining the sum of thethicknesses of the two tubes. By suitable procedure, the thickness ofindividual tubes can obviously be calculated.

The incident beam is weakened in traversing the material by the amountthat is scattered in all directions, and by the amount absorbed in thematerial. The scattered intensity is also weakened by absorption as wellas by rescattering. These factors set an upper limit on the thickness ofany wall which may be accurately determined by this method This upperlimit is almost ontirely determined by the penetrating powers or"hardness" oi: the radiation emitted by the source. Using the gamma raysfrom radium B and radium C in equilibrium with radium, this limitappears to be from three-quarters to one inch of iron, or somewhat morein lighter materials. However, it is emphasized that this method doesnot limit itself to the use of gamma rays, but may make use of anyradiation or penetrating particles such as X-rays, visible light, alphaand beta particles, neutrons, and the like. In fact, it appears thatwith the proper use of fast and slow neutrons, the limit of thicknessmay be increased to as much as three or more inches of iron, thus makingpossible the measurement of walls of considerable thickness. 7

While the invention has been described with reference to measuring thethickness of the walls of vessels, tubes or pipes in plants such as oilrefineries and the like, it is to be understood that the principles arealso applicable to the measuring of the wall thickness of vessels andpipes, such as for instance drill pipe and other tubing to be used inwell production.

Obviously, many other modifications and variations of the invention-ashereinbefore set forth may 'be .made without'departing from-the spiritand scope thereof, and therefore only such limitations should be imposedas are indicated by the appended claims.

I claim:

1. The method of measuring the thickness of a wall from one side thereofwhich comprises directing a beam of penetrative radiation into said wallfrom one side thereof, and determining from the same side of said wallthe amount of radiation scattered in the material of the wall andreturned outwardly of said side.

2. The method of measuring the thickness of a plate or of the wall of atube or the like which comprises passing a beam .of penetrativeradiation into said wall from one side thereof, and determining the.amount of radiation scattered in the material of the wall and returnedto a detector on the same side of said wall as the source of radiation,the amount of said returned radiation being proportional-to thethickness of said wall.

3. The method of measuring the thickness of a wall from one side thereofwhich comprises placing a source of penetrative radiation near thesurface of said wall so that said radiation enters said wall wherein itis scattered and some of the .radiation returned outwardly of said wall,and detecting the amount of said returned radiation by means of adetector placed near said source and at the same side of said wall assaid source.

4. The method of measuring the thickness of a wall from one side thereofwhich comprises directing a beam of penetrative radiation into said wallfrom one side thereof, intercepting a portion of the radiation scatteredin the wall and returned outwardly of said side, directing a similarbeam of radiation into another wall of the same material as said firstwall and of known thickness, intercepting a portion of the radiationscattered within said last mentioned wall and returned outwardlythereof. and comparing the amounts of radiation intercepted from the twowalls. I v

5. A device for determining the thickness of a wall from one sidethereof, comprising a casing adapted to be'piaced in contact with saidside of said wall, a source of penetrative radiation dis posed withinsaid casing, means for directing a beam of said radiation from saidsource to said wall, a detector associated with said casing forintercepting some of said radiation scattered within the material ofsaid wall, a radiation shield member between said source and saiddetector, and means connected to said detector for indicating the amountof scattered radiation detected.

6. A device for determining the thickness of a wall from one sidethereof, comprising a shield member adapted to be placed against oneside of said wall, a source of penetrative radiation disposed withinsaid member, said member being provided with a collimating slot fordirecting a beam of said radiation from said source into said wall,means associated with said shield member for intercepting a portion ofthe radiation scattered in said wall and means connected with said firstmeans for indicating the amount oi scattered radiation intercepted.

7. A device for determining the thickness of a wall from one sidethereof, comprising a lead block adapted to be placed against said sideof said wail, said block being provided with an opening in the sideadjacent the wall, a source of penetrative radiation disposed in saidblock, said block also being provided with a slot connecting said sourcewith said opening, and a device associated with said block for detectingradiation scattered within said wall near said opening and returnedoutwardly of said well at the side where the block is located.

8. A device for determining the thickness of the wall of a tube from theinside thereof, comprising a lead shield member adapted to be placedwithin and against the inner surface of said tube, a source ofpenetrative radiation disposed within said member, said member beingprovided with a. slot for directing a beam of said radiation out throughsaid member and into said wall, and means disposed adjacent said memberfor detecting radiation scattered within said wall and returned throughthe inner surface thereof.

9. A device for determining the thickness of the wall of a tube from theinside thereof, comprising a lead shield member having a portionconforming to the curvature of the inner surface of said tube andadapted to be placed against said surface, said portion being providedwith an opening adapted to be adjacent said inner surface when thedevice is in operating position, a source of penetrative radiationmounted within said member, said member being provided with acollimating slot for directing a beam of radiation from said source tosaid opening and into said wall, and a detector disposed near said memher for intercepting radiation scattered in said wall and returned tothe detector through said inner surface.

10. A device for determining the thickness of the wall of a tube fromthe inside thereof, comprising a lead shield member adapted to be placedwithin and against the inner surface of said tube, a source ofpenetrative radiation disposed within said member, said member beingprovided with a. slot for directing a beam of said radiation out throughsaid member and into said wall, and means disposed adjacent said memberfor detecting radiation scattered within said with and returned throughthe inner surface thereof, means attached to said shield member wherebyit can be moved through said tube and an instrument connected to saiddetecting means for indicating the amount of radiation detected.

DONALD G. C. HARE.

